Transmit beamforming in wireless systems can be performed in either a closed-loop or an open-loop manner. An open-loop system is well suited for Time Division Duplexing (TDD) systems. An open-loop system does not require channel information feedback. As a result, less overhead is required. However, the disadvantage of an open-loop system is that the system needs to constantly conduct phase calibration in order to compensate for the phase difference between the transmission and reception Radio Frequency (RF) chains among the multiple transmit antennas. Another disadvantage of an open-loop system is that the system requires a constant uplink phase reference such as uplink pilots. This requirement could lead to an excessive feedback overhead. The process of phase calibration is generally costly and sensitive to radio channel environment.
A closed-loop system, on the other hand, does not require phase calibration process. However, a closed-loop system does require channel feedback to the transmitter, which results in additional overhead. Furthermore, a closed-loop system is also sensitive to feedback channel error due to feedback delay or fast channel variation. Typically, Frequency Division Duplexing (FDD) systems employ closed-loop transmit beamforming schemes. However, a closed-loop scheme also can be applied to TDD systems.